The present invention relates generally to microwave antennas, and more specifically the invention pertains to an optical feed structure for controlling microwave phased array antennas.
A phased array antenna is a network of radiating elements, having a cooperative radiation pattern that is a highly directive beam. Whereas conventional radar antennae have to be mechanically steered to meet beam directing requirements, a phased array achieves the same effect electronically by changing the phase of the signal radiated by each element. Thus, accurate beams are formed and directed simply by driving each element of the array with a signal having an appropriate phase. As a further advantage, electronic steering is much faster than mechanical steering.
The flexibility of electronic steering provided by phased arrays requires individual control of each element. In an array having N elements, each of the elements is driven with a different phase of the same signal.
Electronically scanned radars include a feed network which couples microwave energy from the transmitter to a radiating aperture of the antenna, as well as from the aperture to the receiver. Feed networks are constructed in a variety of forms, with the corporate feed being particularly useful in providing an accurate distribution of microwave energy across the radiating aperture.
The task of providing an optical system for controlling microwave phased array antennas is alleviated to some extent by the systems disclosed in the following U.S. patents, the disclosure of which are specifically incorporated herein by reference:
U.S. Pat. No. 4,956,603 issued to Hong et al; PA1 U.S. Pat. No. 4,814,773 issued to Wechsberg et al; PA1 U.S. Pat. No. 4,583,096 issued to Bellman, et al; and PA1 U.S. Pat. No. 3,878,520 issued to Wright et al.
The patents identified above relate to fiber optic network apparatus for phased array radar systems. In particular, the Hong et al. patent describes an optical beamforming network for controlling the RF radiation pattern of a phased array antenna. A spatial light modulator is user-programmed with a desired far field radiation footprint, and modulates the light from a laser. The modulated light beam is directed through a Fourier transform lens and onto a beam splitter. The light is then combined with light from a second laser that is frequency offset by the RF center frequency of the antenna. Light from the beam splitter is recovered by first and second fiber optic bundles, and each optical fiber leads to a corresponding photodetector. The outputs of corresponding photodetectors of the two fiber optic bundles are combined to control the radiation of a corresponding radiation element of the phased array.
The Wechsberg et al patent relates to an optical feed system capable of coupling an antenna with transmitting and receiving circuity. The feed system comprises a set of optical multiplexers interconnected by sets of optical fibers. The microwave energy of the radar is converted to optical radiation for communication to the antenna, where it is converted back to microwave energy. Electro-optic modulators and photoelectric detectors provide the energy conversion. A plurality of signals can be simultaneously coupled via the optical fibers by utilization of radiation of differing frequencies.
The Bellman et al patent describes a system for fiber optic distribution of data in which digitally encoded data drives an optical light source which illuminates a bundle of fibers. A fiber from this bundle is terminated in the vicinity of each element of one row of a phased array. A photosensor on a transmit/receive element receives the modulated light signal. A similar but independent light source and fiber optic bundle is provided for every row of the array. Similar sources and fiber optic bundles are independently provided for every individual column of the array.
The Wright et al patent relates to an optically operated microwave phased array antenna system. Two optical beams are generated with a difference frequency equal to the desired microwave frequency to be transmitted. The two beams are combined to produce a two dimensional optical pattern that contains the correct microwave phase and amplitude information to form and steer the final antenna beam in space. The optical pattern is actually an optical analog of the microwave excitation applied to the antenna radiating elements. A transducer system converts the optical pattern to a two dimensional microwave pattern which is a two dimensional array of microwave signals. Each signal is connected to a single radiating element of a phased array antenna. These elements cooperate to radiate a beam in space.
The references described above demonstrate an ongoing need for optical control systems for use with microwave phased array antennas. The present invention includes an optical feed structure that is designed to help satisfy that need for both radar and communication microwave antenna systems.
Microwave beamforming in phased array antennas requires that each element of the array transmit a properly phased microwave signal so that the desired far field beam pattern is created. Conventional electronic methods for phased array feed systems tend to be expensive, bulky, lossy, inefficient, and susceptible to electromagnetic interference. Several other optical implementations for microwave beamforming have been proposed. The present invention does not require numerous lossy optical switches as does the switched fiber approach, nor does it require a segmented mirror device.